Carbonates such as ethylene carbonate, propylene carbonate and dimethyl carbonate are used as solvents for electrolyte salts of lithium secondary batteries, solar cells, radical batteries and capacitors. However, carbonates have a low flash point and high flammability, and may thus involve the danger of ignition and explosion on account of overcharging and overheating. Drops in output derived from higher conductivity and lower conductivity at low temperatures constitute further problems.
Enhancing the withstand voltage of an electrolytic solution in order to achieve higher capacity is a requirement in lithium secondary batteries. In capacitors, preferably, both the negative electrode and the positive electrode are of hard carbon, and it is particularly desirable that the capacitor can be used stably at 3 V or above. However, electrolytic solutions decompose at 3 V or above in the case of solvents conventionally used for electrolyte salts, such as propylene carbonate or dimethyl carbonate, and hence such solvents cannot be used at 3 V or above.
Accordingly, it has been proposed to use fluorine-containing cyclic carbonates as electrolytic solutions. For instance, it has been proposed (Patent Literature 1 to 4) to use a compound wherein some of the hydrogen atoms of ethylene carbonate are substituted with fluorine atoms, or to use a compound wherein some or all of the hydrogen atoms of the methyl group in propylene carbonate are substituted with fluorine atoms (Patent Literature 5 to 6).
However, compounds wherein some of the hydrogen atoms of ethylene carbonate are substituted with fluorine atoms are difficult to synthesize and isolate, and moreover exhibit insufficient flame retardancy. Compounds wherein some or all of the hydrogen atoms of the methyl group of propylene carbonate are substituted with fluorine atoms have been found to give rise to, for instance, low electrolyte salt solubility, lower discharge efficiency, increased viscosity and the like, and thus these compounds did not necessarily satisfy the required performance. Electrolytic solutions in lithium secondary batteries, capacitors and so forth are desirably liquid at −20° C. or above. However, trifluoromethyl cyclic carbonate is solid at −20° C., and the use thereof in these applications is therefore limited.
It is also desirable to enhance not only flame retardancy and withstand voltage, but also low-temperature characteristics, in terms of precluding increases in viscosity even at low temperature, while incurring small drops in conductivity, in electrolytic solutions of capacitors and radical batteries, which undergo repeated charge and discharge as is the case in lithium secondary batteries.
In order to solve the above problem, Patent Literature 7 proposed an electrolytic solution that comprises an electrolyte salt and a fluorine-containing cyclic carbonate that has a fluorine-containing ether group or a fluorine-containing alkyl group having two or more carbon atoms.
Also, Non Patent Literature 1 and 2 describe the electrochemical behavior, thermal stability and electrochemical characteristics, in lithium ion batteries, of organo-fluorine compounds that include a fluorine-containing cyclic carbonate that has a fluorine-containing ether group or a fluorine-containing alkyl group having two or more carbon atoms.